Optical analyses of fluids, including gases, are well known, and various optical and spectroscopic techniques have been applied in oilfield environments to analyze formation fluids. For example, U.S. Pat. No. 4,994,671 to Safinya et al. describes an apparatus and method for analyzing the composition of formation fluids. Formation fluids are drawn into a testing region and analyzed by directing light at the fluids and detecting the spectrum of transmitted and/or scattered light. The detected spectra are fit to spectra of known composition to determine the composition of the fluid sample. U.S. Pat. No. 5,266,800 to Mullins and U.S. Pat. No. 5,331,156 to Hines et al. describe applying optical density measurements to distinguish between crude oils and to analyze water and oil fractions, respectively, in, e.g., a formation flow stream obtained by a borehole tool. U.S. Pat. Nos. 5,167,149 to Mullins et al. and 5,201,220 to Mullins et al. describe a method and apparatus that involve transmitting light towards a fluid in a flow line and detecting reflected light at various angles of incidence. Information related to the Brewster angle and critical angle of in known gas volumes of formation fluids is used to categorize the fluid in the flow line as high gas, medium gas, low gas, and no gas. U.S. Pat. No. 5,859,430 to Mullins et al. describes a borehole tool and method for the downhole analysis of formation gases. When substantial amounts of gas are detected in a fluid stream, the fluid stream is diverted into a sample cell. The gaseous fluid sample is analyzed by directing light to the sample cell and detecting absorbance spectra. The detected spectra are fit to known spectra of various hydrocarbons in order to obtain information regarding the hydrocarbon composition in the gas stream.
U.S. Pat. No. 4,994,671, No. 5,266,800, No. 5,331,156, No. 5,167,149, No. 5,201,220, and No. 5,859,430 are each incorporated by reference herein in their entireties.
The invention provides methods of detecting carbon dioxide downhole in, for example, a borehole, a production well, or other subsurface earth formation. In one aspect, the invention provides a method of distinguishing between carbon dioxide and methane downhole. Near-infrared light is transmitted through formation gases downhole, and indications of near-infrared absorptions are detected from the formation gases. The detected indications of near-infrared absorptions are used to distinguish carbon dioxide from methane in the formation gases.
Another aspect of the invention provides a method of monitoring for carbon dioxide breakthrough in an enhanced oil recovery operation. An evaluation tool is inserted into a stream of fluid flowing in a wellbore. When gas is determined to be present in the fluid stream, near-infrared light is transmitted through the gas and indications of near-infrared absorptions are detected from the gas. The indications of near-infrared absorptions are used to determine the presence of carbon dioxide in the stream of fluid.
Further details and features of the invention will become more readily apparent from the detailed description that follows.